1. Field of the Invention
The present invention relates to novel heat-resistant polyamides having specific heat resistance and moldability or solubility features, and useful as a base material for fibers, films, and moldings such as injection moldings.
2. Description of the Related Art
When aliphatic polyamides are formed into fibers, they are considered to be excellent fibers for use in, for example, clothing. But, aliphatic polyamide fibers, although having a great elongation, have a lower strength and modulus, and an inferior heat resistance, compared with aromatic polyamide fibers, and therefore, the uses thereof are limited.
Aromatic polyamides, however, have a higher softening point and melting point, and an extremely good heat resistance, such as strength maintenance at high temperature, and mechanical characteristics such as strength and modulus. But, when formed into fibers, the elongation is as low as less than 5%, and there is a drawback in that the fiber itself may be fibrillated and fractured into a number of strings because the fiber is produced by liquid crystal spinning of an aromatic polyamide.
Aramide, which is a typical all aromatic polyamide fiber, is known as a polyamide useful for the formation of a high tenacity fiber, having a prolonged heat-resistant temperature of about 220.degree. C. When this aramide is formed into fibers, however, it must be first made into a solution. Such a solution has been prepared by using dimethylacetamide or hexamethylenephosphoramide containing lithium chloride or calcium chloride suspended therein as the solvent, but the suspended lithium chloride or calcium chloride may cause clogging in a pipe, thereby resulting in production problems such as equipment failure.
Also, when manufacturing molded products, it is difficult to carry out injection molding due to the high heat resistance of the aramide. Thus, this aromatic polyamide is difficult to work, in spite of a high performance in general.
Korshak et al reported a soluble polyamide in the Journal of Macromolecule Science (J. Macromol. Sci., Rev. Macromol. Chem., C11, 45, 1974), but an example having both solubility and moldability, while maintaining the heat resistance at a high level, is not disclosed in this report.
Generally speaking, an aromatic polyamide having a high rigidity and asymmetry has excellent mechanical properties but has a high melting point, which is approximate to the decomposition point, and therefore, melt molding of this polyamide is difficult. Further, this polyamide has an inferior solubility, and thus cannot be easily used as an industrial material.
As mentioned above, poly(p-phenyleneterephthalamide), which is a typical aromatic polyamide, is soluble in conc. sulfuric acid or hexamethylphosphorylamide or N-methylpyrrolidone containing lithium chloride, calcium chloride, etc., dissolved therein, but still has a low solubility and, therefore, is difficult to use as a solution.
On the other hand, an aliphatic polyamide has an inferior heat resistance, and even a polyterephthalic amide partially converted to an aromatic does not have sufficient heat resistance. For example, when hexamethylenediamine or propylenediamine is used as the amine component, problems of not only heat resistance but also solubility arise.
Also, although an improvement of the solubility makes film or fiber working easier, it does not assist the working of a molded product for machinery and electrical parts. Usually, the injection molding of a synthetic resin can be widely used, but in this case, the heat resistance of the synthetic resin poses a problem.
If the heating temperature during injection molding is elevated, the synthetic resin will undergo thermal decomposition, lowering the physical properties of the molded product. Therefore, the lower limit of the molding temperature for crystalline synthetic resins in general is accepted to be preferably higher by about 30.degree. C. than the melting point (Tm) thereof, and for amorphous polymers, higher by about 100.degree. C. than the glass transition point (Tg). In either case, the synthetic resin temperature during injection molding must be at a temperature which is approximately 50.degree. C. or more lower than the thermal decomposition temperature of the synthetic resin.
In other words, if the upper limit of the heating temperature in injection molding is 400.degree. C., as required by the equipment, the upper limit of the Tm is 370.degree. C. in crystalline polymers and the upper limit of the Tg is 300.degree. C. in amorphous polymers.